Process for the preparation of a two-coat finish, and aqueous coating materials

ABSTRACT

The invention relates to a process for the preparation of a two-coat finish on a substrate surface, in which process a basecoat is employed which contains as binder a polymer which can be obtained by subjecting an ethylenically unsaturated monomer or a mixture of ethylenically unsaturated monomers to free-radical polymerization in an aqueous dispersion of a polyurethane resin which has a number-average molecular weight of from 1000 to 30,000 and contains on average from 0.05 to 1.1 polymerizable double bonds per molecule, and in the presence of a water-insoluble initiator or of a mixture of water-insoluble initiators, the weight ratio of the polyurethane resin to the ethylenically unsaturated monomer or to the mixture of ethylenically unsaturated monomers being between 1:10 and 10:1.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a process for the preparation of a two-coatfinish on a substrate surface, in which

(1) a pigmented aqueous basecoat is applied to the substrate surface,

(2) a polymer film is formed from the basecoat applied in step (1),

(3) a transparent topcoat is applied to the basecoat obtained in thisway, and subsequently

(4) the basecoat and the topcoat are baked together.

The invention also relates to aqueous coating materials which can beemployed in this process as pigmented basecoats.

The process described above is employed in particular for thepreparation of two-coat automotive finishes of the basecoat/clearcoattype. In this context the quality of the two-coat finish prepared bythis process depends quite critically on the aqueous basecoat employedin step (1) of the process.

EP-A-353 797 describes aqueous coating materials which can be employedas basecoats in step (1) of the above-described process. The aqueouscoating materials described in EP-A-353 797 contain as binder a polymerwhich is obtainable by subjecting acrylate and/or methacrylate monomersto an emulsion polymerization which is initiated by water-solubleinitiators, in the presence of an anionic polyurethane resin which mayalso contain vinyl groups.

If the aqueous coating materials described in EP-A-353 797 are employedas basecoats in the above-described process, two-coat finishes areobtained which have an unsatisfactory stability with respect tocondensed moisture. This disadvantage manifests itself in particular inrefinishes, which are only cured at temperatures up to 80° C. Moreover,the aqueous coating materials described in EP-A-353 797 have anunsatisfactory storage stability if they contain a melamine resin asadditional binder component.

EP-A-297 576 describes a process for the preparation of two-coatfinishes of the type described above, in which the aqueous coatingmaterials employed as basecoat contain an aqueous polymer dispersionwhich is obtainable by polymerizing ethylenically unsaturated monomersin an aqueous dispersion in the presence of a polyurethane resin whichcontains urea groups but no vinyl groups. If the aqueous coatingmaterials described in EP-A-297 576 are employed as basecoats in theabove-described process for the preparation of two-coat finishes, thenthe two-coat finishes obtained are in need of improvement with respectto their resistance to condensed water. Moreover, the aqueous coatingmaterials described in EP-A-297 576 often display deficiencies instorage stability, and defects which can be traced back to instances ofincompatibility, if combinations of different binders are employed.

DE-A-40 10 176 describes a process for the preparation of a two-coatfinish, in which an aqueous basecoat is employed which contains asbinder a polymer which is obtainable by polymerizing ethylenicallyunsaturated monomers in an organic solvent in the presence of apolyurethane resin which contains polymerizable double bonds andconverting the resulting reaction product to an aqueous dispersion. Ifbasecoats which contain metal pigments are employed in the processdescribed in DE-A-40 10 176, the two-coat metallic finishes obtained arein need of improvement with respect to their metallic effect. Moreover,a disadvantage in the preparation of the aqueous basecoats described inDE-A-40 10 176 is that that a large quantity of organic solvents isrequired.

The object of the present invention was to provide a new process for thepreparation of two-coat finishes of the type described above, with whichtwo-coat finishes are obtained whose properties are improved incomparison with the prior art and in which, in particular, theabove-described disadvantages of the prior art are absent or lessened.

This object is surprisingly achieved by a process for the preparation ofa two-coat finish on a substrate surface, which process consists ingoing through the steps (1) to (4) described above and is characterizedin that the basecoat contains as binder a polymer which can be obtainedby subjecting an ethylenically unsaturated monomer or a mixture ofethylenically unsaturated monomers to free-radical polymerization in anaqueous dispersion of a polyurethane resin which has a number-averagemolecular weight of from 1000 to 30,000 and contains on average from0.05 to 1.1 polymerizable double bonds per molecule, and in the presenceof a water-insoluble initiator or of a mixture of water-insolubleinitiators, the weight ratio of the polyurethane resin to theethylenically unsaturated monomer or to the mixture of ethylenicallyunsaturated monomers being between 1:10 and 10:1.

In cases where a basecoat is employed which contains metal pigment thetwo-coat finishes prepared by the process according to the inventionexhibit an excellent metallic effect. The adhesion between basecoat andsubstrate and the adhesion between basecoat and clearcoat is excellent.Moreover, the two-coat finishes prepared by the process according to theinvention exhibit a very good resistance to high atmospheric humidity.The aqueous basecoats employed in accordance with the invention arestable on storage and exhibit no defects which can be traced back toincompatibility phenomena, even when combinations of different bindersare employed.

DETAILED DESCRIPTION

The aqueous dispersion of the polyurethane resin, in which theethylenically unsaturated monomer or the mixture of ethylenicallyunsaturated monomers is subjected to free-radical polymerization in thepresence of a water-insoluble initiator or of a mixture ofwater-insoluble initiators, can be prepared by using, from

(a) a polyester- and/or polyether-polyol having a number-averagemolecular weight of from 400 to 5000 or a mixture of such polyester-and/or polyether-polyols, and

(b) a polyisocyanate or a mixture of polyisocyanates, together ifdesired with a monoisocyanate or a mixture of monoisocyanates, and

(c) a compound which contains at least one group which is reactivetoward isocyanate groups and at least one group which is capable offorming anions in the molecule, or a mixture of such compounds, or

(d) a compound which contains at least one group which is reactivetoward NCO groups and at least one poly(oxyalkylene) group in themolecule, or a mixture of such compounds, or

(e) a mixture of components (c) and (d), and

(f) if desired, a compound which contains not only a polymerizabledouble bond but also at least one group which is reactive toward NCOgroups, or a mixture of such compounds, and

(g) if desired, an organic compound which contains hydroxyl and/or aminogroups and has a molecular weight from 60 to 399, or a mixture of suchcompounds,

to prepare a polyurethane resin which has a number-average molecularweight of from 1000 to 30,000, preferably from 1500 to 20,000, andcontains on average from 0.05 to 1.1, preferably from 0.2 to 0.9,polymerizable double bonds, and by dispersing this resin in water.

The polyurethane resin can be prepared both in bulk and in organicsolvents.

The polyurethane resin can be prepared by simultaneous reaction of allthe starting compounds. However, in many cases it is advantageous toprepare the polyurethane resin in steps. For example, it is possible toprepare an isocyanato-containing prepolymer from components (a) and (b),which is then reacted further with component (c) or (d) or (e).Furthermore, it is possible to prepare an isocyanato-containingprepolymer from components (a) and (b) and (c) or (d) or (e) and, ifdesired, (f), which can then be reacted with component (g) to give ahigher molecular weight polyurethane resin. The reaction with component(g) may be carried out in bulk or--as described for example in EP-A-297576--in water. In cases where the compound employed as component (f)contains only one group which is reactive toward isocyanate groups, in afirst step an isocyanato-containing precursor can be prepared from (b)and (f), which can subsequently be reacted further with the othercomponents.

The reaction of components (a) to (g) can also be carried out in thepresence of catalysts such as, for example, dibutyltin dilaurate,dibutyltin maleate and tertiary amines.

The quantities of component (a), (b), (c), (d), (e), (f) and (g) to beemployed are determined by the number-average molecular weight to beaimed for and the acid number to be aimed for. The polymerizable doublebonds can be introduced into the polyurethane molecules by employingcomponents (a) which contain polymerizable double bonds and/orcomponents (b) which contain polymerizable double bonds, and/orcomponent (f). It is preferred to introduce the polymerizable doublebonds by way of component (f). It is further preferred for the groupswhich contain polymerizable double bonds and are introduced into thepolyurethane resin molecules to be acrylate, methacrylate or allyl ethergroups.

As component (a) it is possible to employ saturated and unsaturatedpolyester- and/or polyether-polyols, especially polyester- and/orpolyether-diols having a number-average molecular weight of from 400 to5000. Examples of suitable polyether-diols are polyether-diols of thegeneral formula H(--O--(CHR¹)_(n) --)_(m) OH, in which R¹ is hydrogen ora lower substituted or unsubstituted alkyl radical, n is 2 to 6,preferably 3 to 4, and m is 2 to 100, preferably 5 to 50. Examples arelinear or branched polyether-diols such as poly(oxyethylene) glycols,poly(oxypropylene) glycols and poly(oxybutylene) glycols. Thepolyether-diols selected should not contribute excessive quantities ofether groups, since otherwise the polymers formed swell in water. Thepreferred polyether-diols are poly(oxypropylene) glycols in themolecular mass range M_(n) of from 400 to 3000.

Polyester-diols are prepared by esterification of organic dicarboxylicacids or their anhydrides with organic diols or are derived from ahydroxycarboxylic acid or from a lactone. In order to prepare branchedpolyester-polyols it is possible to a small extent to employ polyols orpolycarboxylic acids of a higher functionality. The dicarboxylic acidsand diols may be linear or branched aliphatic, cycloaliphatic oraromatic dicarboxylic acids or diols.

The diols used to prepare the polyesters consist, for example, ofalkylene glycols such as ethylene glycol, propylene glycol, butyleneglycol, 1,4-butanediol, 1,6-hexanediol, neopentylglycol and other diolssuch as dimethylolcyclohexane. However, it is also possible to add smallquantities of polyols such as trimethylolpropane, glycerol andpentaerythritol. The acid component of the polyester primarily compriseslow molecular weight dicarboxylic acids or their anhydrides having from2 to 44, preferably from 4 to 36, carbon atoms in the molecule. Examplesof suitable acids are o-phthalic acid, isophthalic acid, terephthalicacid, tetrahydrophthalic acid, cyclohexanedicarboxylic acid, succinicacid, adipic acid, azelaic acid, sebacic acid, maleic acid, fumaricacid, glutaric acid, hexachloroheptanedicarboxylic acid,tetrachlorophthalic acid and/or dimerized fatty acids. Instead of theseacids it is also possible to use their anhydrides, provided the latterexist. In the formation of polyester-polyols it is also possible forminor quantities of carboxylic acids having 3 or more carboxyl groups tobe present, for example trimellitic anhydride or the adduct of maleicanhydride with unsaturated fatty acids.

It is also possible to employ polyester-diols which are obtained byreacting a lactone with a diol. They are distinguished by the presenceof terminal hydroxyl groups and repeating polyester units of the formula(--CO--(CHR²)_(n) --CH₂ --O). In this formula n is preferably 4 to 6 andthe substituent R² =hydrogen or an alkyl, cycloalkyl or alkoxy radical.No substituent contains more than 12 carbon atoms. The total number ofcarbon atoms in the substituent does not exceed 12 per lactone ring.Examples of such compounds are hydroxycaproic acid, hydroxybutyric acid,hydroxydecanoic acid and/or hydroxystearic acid.

For the preparation of the polyester-diols preference is given to theunsubstituted epsilon-caprolactone, in which n has the value 4 and allsubstituents R² are hydrogen. The reaction with lactone is initiatedwith low molecular weight polyols such as ethylene glycol,1,3-propanediol, 1,4-butanediol and dimethylolcyclohexane. However, itis also possible to react other reaction components with caprolactone,such as ethylenediamine, alkyldialkanolamines or else urea. Othersuitable high molecular weight diols are polylactamdiols, which areprepared by reaction of, for example, epsilon-caprolactam with lowmolecular weight diols.

If the intention is to introduce polymerizable double bonds into thepolyurethane molecules by way of component (a), then components (a) mustbe employed which contain polymerizable double bonds. Examples of suchcomponents (a) are polyester-polyols, preferably polyester-diols, whichhave been prepared using polyols or polycarboxylic acids which containpolymerizable double bonds, preferably polyols which containpolymerizable double bonds. Examples of polyols which containpolymerizable double bonds are trimethylolpropane monoallyl ether,glycerol monoallyl ether, pentaerythritol monoallyl ether andpentaerythritol diallyl ether.

As component (b) it is possible to employ aliphatic and/orcycloaliphatic and/or aromatic polyisocyanates. Examples of aromaticpolyisocyanates are phenylene diisocyanate, tolylene diisocyanate,xylylene diisocyanate, biphenylene diisocyanate, naphthylenediisocyanate and diphenylmethane diisocyanate.

Because of their good resistance to ultraviolet light, (cyclo)aliphaticpolyisocyanates give products with a low tendency to yellowing. Examplesof cycloaliphatic polyisocyanates are isophorone diisocyanate,cyclopentylene diisocyanate and the hydrogenation products of thearomatic diisocyanates, such as cyclohexylene diisocyanate,methylcyclohexylene diisocyanate and dicyclohexylmethane diisocyanate.Aliphatic diisocyanates are compounds of the formula

    OCN--(CR.sup.3.sub.2).sub.r --NCO

in which r is an integer from 2 to 20, in particular 6 to 8, and R³,which may be identical or different, is hydrogen or a lower alkylradical of 1 to 8 carbon atoms, preferably 1 or 2 carbon atoms. Examplesof such compounds are trimethylene diisocyanate, tetramethylenediisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate,propylene diisocyanate, ethylethylene diisocyanate, dimethylethylenediisocyanate, methyltrimethylene diisocyanate and trimethylhexanediisocyanate. A further example of an aliphatic diisocyanate istetramethylxylene diisocyanate.

As diisocyanates it is particularly preferred to employ hexamethylenediisocyanate, isophorone diisocyanate, tetramethylxylene diisocyanateand dicyclohexylmethane diisocyanate.

With respect to the functionality of the polyisocyanates, component (b)must have a composition such that no crosslinked polyurethane resin isobtained. In addition to diisocyanates, component (b) may also contain aproportion of polyisocyanates with functionalities of more than two--forexample triisocyanates.

Products which have proven suitable as triisocyanates are those formedby trimerization or oligomerization of diisocyanates or by reaction ofdiisocyanates with polyols or polyamines. Examples of these include thebiuret of hexamethylene diisocyanate and water, the isocyanurate ofhexamethylene diisocyanate or the adduct of isophorone diisocyanate withtrimethylolpropane. If desired, the average functionality can be loweredby addition of monoisocyanates. Examples of such chain-terminatingmonoisocyanates are phenyl isocyanate, cyclohexyl isocyanate,1-(1-isocyanato-1-methylethyl)-3-(1-methylethenyl) benzene[sic] andstearyl isocyanate.

To enable the polyurethane resin under discussion to be converted to astable dispersion in water it must contain hydrophilic groups. Thesehydrophilic groups are introduced into the polyurethane resin by way ofcomponent (c) or component (d) or component (e). The groups of component(c) which are capable of forming anions are neutralized, prior to orduring the dispersion of the polyurethane resin in water, with a base,preferably a tertiary amine, for example dimethylethanolamine,triethylamine, tripropylamine and tributylamine, so that thepolyurethane resin contains anionic groups after the neutralization. Inthe case where component (c) is the only component employed whichsupplies hydrophilic groups, component (c) is employed in a quantitysuch that the polyurethane resin has an acid number of from 15 to 80,preferably from 20 to 60. In the case where component (d) is the onlycomponent employed which supplies hydrophilic groups, component (d) isemployed in a quantity such that the polyurethane resin contains from 5to 40% by weight, preferably from 10 to 30% by weight, of oxyalkylenegroups, any oxyalkylene groups introduced by component (a) beingincluded in the calculation. In the case where component (e) is employedas a component which supplies hydrophilic groups, the quantities ofcomponent (c) and (d) to be employed in accordance with the mixing ratioare between the values indicated above for the cases where component (c)or (d) is employed as the sole supplier of hydrophilic groups. Besidesthis, the person skilled in the art can readily determine the quantitiesof component (c), (d) or (e) to be employed by means of simple routineexperiments. All that they have to do is to test, by means of series ofsimple experiments, how high the proportion of hydrophilic groups mustat least be in order to obtain a stable aqueous polyurethane resindispersion. In addition, they can of course also make use of generallyconventional dispersion auxiliaries, for example emulsifiers, in orderto stabilize the polyurethane resin dispersions. The use of dispersionauxiliaries, however, is not preferred, since it generally increases thesensitivity to moisture of the finishes obtained.

As component (c) it is preferred to employ compounds which contain twogroups in the molecule which are reactive toward isocyanate groups.Suitable groups which are reactive toward isocyanate groups are, inparticular, hydroxyl groups, and also primary and/or secondary aminogroups. Suitable groups which are capable of forming anions arecarboxyl, sulfonic acid and/or phosphonic acid groups, with carboxylgroups being preferred. As component (c) it is possible, for example, toemploy alkanoic acids having two substituents on the a carbon atom. Thesubstituent may be a hydroxyl group, an alkyl group or, preferably, analkylol group. These alkanoic acids have at least one, generally from 1to 3, carboxyl groups in the molecule. They have from 2 to about 25,preferably from 3 to 10, carbon atoms. Examples of component (c) aredihydroxypropionic acid, dihydroxysuccinic acid and dihydroxybenzoicacid. A particularly preferred group of alkanoic acids are theα,α-dimethylolalkanoic acids of the general formula

    R.sup.4 --C(CH.sub.2 0H).sub.2 COOH, in which

R⁴ is a hydrogen atom or an alkyl group having up to about 20 carbonatoms.

Examples of such compounds are 2,2-dimethylolacetic acid,2,2-dimethylolpropionic acid, 2,2-dimethylolbutyric acid and2,2-dimethylolpentanoic acid. The preferred dihydroxyalkanoic acid is2,2-dimethylolpropionic acid. Examples of compounds containing aminogroups are α,w-diaminovaleric acid, 3,4-diaminobenzoic acid,2,4-diaminotoluenesulfonic acid and 2,4-diaminodiphenyl-ether-sulfonicacid.

Using component (d), it is possible to introduce poly(oxyalkylene)groups, as nonionic stabilizing groups, into the polyurethane molecules.Examples of component (d) which can be employed arealkoxypoly(oxyalkylene) alcohols having the general formula R' O--(--CH₂--CHR"--O--)_(n) H, in which R' is an alkyl radical having 1 to 6 carbonatoms, R" is a hydrogen atom or an alkyl radical having 1 to 6 carbonatoms and n is a number between 20 and 75.

Component (f) is used for introducing polymerizable double bonds intothe polyurethane resin molecules. It is preferred to employ as component(f) a compound which contains at least one group which is reactivetoward NCO groups, and a polymerizable double bond. Particularpreference is given to the employment, as component (f), of compoundswhich contain not only a polymerizable double bond but also two groupswhich are reactive toward NCO groups. Examples of groups which arereactive toward NCO groups are --OH, --SH, >NH and --NH₂ groups, with--OH,>NH and NH₂ groups being preferred. Examples of compounds which canbe employed as component (f) are hydroxy (meth)acrylates, especiallyhydroxyalkyl (meth)acrylates such as hyroxyethyl, hydroxypropyl,hydroxybutyl or hydroxyhexyl (meth)acrylate and 2,3-dihydroxypropyl(meth)acrylate, 2,3-dihydroxypropyl monoallyl ether, allyl2,3-dihydroxypropanoate, glycerol mono(meth)acrylate, glycerol monoallylether, pentaerythritol mono(meth)acrylate, pentaerythritoldi(meth)acrylate, pentaerythritol monoallyl ether, pentaerythritoldiallyl ether, trimethylolpropane monoallyl ether, trimethylolpropanemono(meth)acrylate and trimethylolpropane diallyl ether. As component(f) it is preferred to employ trimethylolpropane monoallyl ether,glycerol mono(meth)acrylate, pentaerythritol di(meth)acrylate,pentaerythritol diallyl ether, glycerol monoallyl ether andtrimethylolpropane mono(meth)acrylate. As component (f) it isparticularly preferred to employ trimethylolpropane monoallyl ether,glycerol monoallyl ether and allyl 2,3-dihydroxypropanoate. It ispreferred to incorporate components (f) which contain at least twogroups which are reactive toward NCO groups into the polyurethanemolecules in chain positions (not terminally).

As component (g) it is possible, for example, to employ polyols havingup to 36 carbon atoms per molecule, such as ethylene glycol, diethyleneglycol, triethylene glycol, 1,2-propanediol, 1,3-propanediol,1,4-butanediol, 1,2-butylene glycol, 1,6-hexanediol, trimethylolpropane,castor oil or hydrogenated castor oil, ditrimethylolpropane ether,pentaerythritol, 1,2-cyclohexanediol, 1,4-cyclohexanedimethanol,bisphenol A, bisphenol F, neopentylglycol, neopentylglycolhydroxypivalate, hydroxyethylated or hydroxypropylated bisphenol A,hydrogenated bisphenol A and mixtures thereof. The polyols are generallyemployed in quantities of up to 30 percent by weight, preferably from 2to 20 percent by weight, based on the quantity of component (a) and (g)employed.

As component (g) it is also possible to employ di- and/or polyaminescontaining primary and/or secondary amino groups. Polyamines areprimarily alkylene-polyamines having 1 to 40 carbon atoms. They maycarry substituents which do not have any hydrogen atoms which arereactive with isocyanate groups. Examples are polyamines having linearor branched, aliphatic, cycloaliphatic or aromatic structure and atleast two primary amino groups. Diamines which can be mentioned arehydrazine, ethylenediamine, propylenediamine, 1,4-butylenediamine,piperazine, 1,4-cyclohexyldimethylamine, 1,6-hexamethylenediamine,trimethylhexamethylenediamine, menthanediamine, isophoronediamine,4,4'-diaminodicyclohexylmethane and aminoethylethanolamine. Preferreddiamines are hydrazine, alkyl- or cycloalkyldiamines such aspropylenediamines and 1-amino-3-aminomethyl-3,5,5-trimethylcyclohexane.It is also possible to employ as component (g) polyamines which containmore than two amino groups in the molecule. In these cases, however, itshould be ensured that--for example by using monoamines as well--that nocrosslinked polyurethane resins are obtained. Polyamines of this kindwhich can be used are diethylenetriamine, triethylenetetramine,dipropylenetriamine and dibutylenetriamine. An example of a monoamine isethylhexylamine.

The binder present in the basecoats employed in accordance with theinvention can be obtained by subjecting an ethylenically unsaturatedmonomer or a mixture of ethylenically unsaturated monomers tofree-radical polymerization in the aqueous polyurethane resin dispersiondescribed above, in the presence of a water-insoluble initiator or of amixture of water-insoluble initiators, the weight ratio of thepolyurethane resin to the ethylenically unsaturated monomer or to themixture of ethylenically unsaturated monomers being between 1:10 and10:1, preferably between 1:2 and 2:1.

Ethylenically unsaturated monomers which can be employed are:

(i) aliphatic or cycloaliphatic esters of acrylic acid or methacrylicacid, which contain neither hydroxyl nor carboxyl groups, or a mixtureof such esters, and

(ii) ethylenically unsaturated monomers which carry at least onehydroxyl group in the molecule, or a mixture of such monomers, and

(iii) ethylenically unsaturated monomers which carry at least onecarboxyl group in the molecule, or a mixture of such monomers, and

(iv) other ethylenically unsaturated monomers, which are different from(i), (ii) and (iii), or a mixture of such monomers, and

(v) polyunsaturated monomers, especially ethylenically polyunsaturatedmonomers, and mixtures of components (i), (ii), (iii), (iv) and (v).

As ethylenically unsaturated monomers it is preferred to employ mixtureswhich range from 40 to 100% by weight, preferably from 60 to 90% byweight, of component (i), from 0 to 30% by weight, preferably from 0 to25% by weight, of component (ii), from 0 to 10% by weight, preferablyfrom 0 to 5% by weight, very particularly preferably 0% by weight ofcomponent (iii) and from 0 to 50% by weight, preferably from 0 to 30% byweight of component (iv) and also from 0 to 5% by weight, preferably 0%by weight, of component (v), the sum of the proportions by weight of(i), (ii), (iii), (iv) and (v) always being 100% by weight.

As component (i) it is possible for example to employ cyclohexylacrylate, cyclohexyl methacrylate, alkyl acrylates and alkylmethacrylates having up to 20 carbon atoms in the alkyl radical, such asmethyl, ethyl, propyl, butyl, hexyl, ethylhexyl, stearyl and laurylacrylate and methacrylate, or mixtures of these monomers.

As component (ii) it is possible for example to employ hydroxyalkylesters of acrylic acid, methacrylic acid or of another α,β-ethylenicallyunsaturated carboxylic acid. These esters may be derived from analkylene glycol which is esterified with the acid, or they may beobtained by reacting the acid with an alkylene oxide. As component (ii)it is preferred to employ hydroxyalkyl esters of acrylic acid andmethacrylic acid, in which the hydroxyalkyl group contains up to 6carbon atoms, or mixtures of these hydroxyalkyl esters. Examples of suchhydroxyalkyl esters are 2-hydroxyethyl acrylate, 2-hydroxypropylacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl acrylate,3-hydroxypropyl methacrylate, 2-hydroxyethyl methacrylate,3-hydroxybutyl (meth)acrylate or 4-hydroxybutyl (meth)acrylate.Corresponding esters of other unsaturated acids such as ethacrylic acid,crotonic acid and similar acids having up to about 6 carbon atoms permolecule can also be employed.

As component (iii) it is preferred to employ acrylic acid and/ormethacrylic acid. However, it is also possible to employ otherethylenically unsaturated acids having up to 6 carbon atoms in themolecule. Examples of such acids are ethacrylic acid, crotonic acid,maleic acid, fumaric acid and itaconic acid.

As component (iv) it is possible for example to employ aromatic vinylhydrocarbons, such as styrene, α-alkylstyrene and vinyltoluene,acrylamide and methacrylamide and acrylonitrile and methacrylonitrile,or mixtures of these monomers.

As component (v) it is possible to employ compounds which contain in themolecule at least two double bonds which can be subjected tofree-radical polymerization. Examples are divinylbenzene,p-methyldivinylbenzene, o-nonyldivinylbenzene, ethanedioldi(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanedioldi(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritoldi(meth)acrylate, allyl methacrylate, diallyl phthalate, butanedioldivinyl ether, divinylethyleneurea, divinylpropyleneurea, diallylmaleate, etc.

Examples of water-insoluble initiators which can be employed arewater-insoluble azo compounds and water-insoluble peroxy compounds.Examples of water-insoluble azo compounds are2,2'-azo-bis(isobutyronitrile), 2,2'-azo-bis-(isovaleronitrile),1,1'-azo-bis-(cyclohexanecarbonitrile) and2,2'-azo-bis-(2,4-dimethylvaleronitrile). Examples of water-insolubleperoxy compounds are t-amyl peroxyethylhexanoate, t-butylperoxyethylhexanoate, dilauryl peroxide, dibenzoyl peroxide and1,1-dimethyl-3-hydroxy-1-butyl peroxyethylhexanoate.

It is of course also possible to add polymerization regulators.

The polymerization of the ethylenically unsaturated monomer or of themixture of ethylenically unsaturated monomers can be carried out byslowly adding the ethylenically unsaturated monomer or the mixture ofethylenically unsaturated monomers to the aqueous polyurethane resindispersion. In this case it is possible both to add the entire quantityof monomers in one go and to introduce only a portion as initial chargeand meter in the remainder during the reaction. However, the monomers tobe polymerized can also be made into a preemulsion using a portion ofthe polyurethane resin dispersion and water, which preemulsion is thenslowly added to the initial charge. The feed time for the monomers to bepolymerized is in general 2-8 hours, preferably about 3-4 hours.

The water-insoluble initiators can be added to the initial charge oradded dropwise together with the monomers. They can also be addedproportionately to the initial charge, which contains a portion of themonomers. The remainder of initiator is then metered in with the rest ofthe monomers. The reaction temperature is determined by the rate ofdissociation of the initiator or initiator mixture and can be reduced,if desired, by means of suitable organic redox systems. Thepolymerization of the ethylenically unsaturated monomer or of themixture of ethylenically unsaturated monomers is in general carried outat a temperature of from 30 to 100° C., in particular at a temperatureof from 60 to 95° C. If it is carried out at superatmospheric pressurethe reaction temperatures may rise to more than 100° C.

The ethylenically unsaturated monomer or the mixture of ethylenicallyunsaturated monomers should be selected such that the binders obtainedin the manner described above have a hydroxyl number of 0-100,preferably 0-80, and an acid number of 10-40, preferably 15-30.

Aqueous binder dispersions prepared in the manner described above can beused by the person skilled in the art to prepare aqueous coatingmaterials which can be employed as basecoats in the above-describedprocess for the preparation of two-coat finishes. In this context it isirrelevant whether the process concerned is a process for theproduction-line preparation of a two-coat finish or is a process for thepreparation of a two-coat finish for repair purposes.

In step (3) of the process described above it is possible in principleto use any transparent topcoats which are suitable for this process, forexample conventional transparent topcoats based on organic solvents,aqueous transparent topcoats or transparent powder coatings.

The aqueous basecoats employed in accordance with the invention may alsocontain, in addition to the binder employed in accordance with theinvention, other water-dilutable synthetic resins such as, for example,amino resins, polyurethane resins, polyacrylate resins, polyesterresins, etc. A particular advantage of the aqueous basecoats employed inaccordance with the invention is that the binder employed in accordancewith the invention is readily compatible with numerous additionallyemployed binders, for example amino resins and polyester resins.

Pigments which may be present in the basecoats employed in accordancewith the invention are color pigments based on inorganic compounds, forexample titanium dioxide, iron oxide, carbon black, etc., color pigmentsbased on organic compounds, and conventional metallic pigments (e.g.commercially available aluminum bronzes, stainless-steel bronzes . . . )and nonmetallic effect pigments (e.g. pearl luster or interferencepigments). The level of pigmentation is within conventional ranges. Thebasecoats according to the invention preferably contain at least onemetallic pigment and/or one nonmetallic effect pigment.

Using the aqueous coating materials according to the invention it isalso possible to prepare high-quality finishes without applying atransparent topcoat on top.

The aqueous coating materials according to the invention can be appliedto any desired substrates, examples being metal, wood, plastic or paper.Application may be made directly or, as is conventional in theautomotive industry, following application of an electrodepositionprimer and a filler.

The aqueous coating materials according to the invention can be appliedby spraying, knife coating, dipping, rolling, and preferably byelectrostatic and pneumatic spraying. In the Examples which follow theinvention is illustrated in more detail. All percentages and parts areby weight unless expressly stated otherwise.

1. Preparation of a Binder Dispersion According to the Invention

178.5 g of a linear polyester (composed of dimerized fatty acid (Pripol®1013), isophthalic acid and 1,6-hexanediol) having a hydroxyl number of80 and a number-average molecular weight of 400 are dissolved, afteraddition of 20.8 g of dimethylolpropionic acid and 7.4 g oftrimethylolpropane monoallyl ether, in 44.6 g of N-methylpyrrolidone and80.9 g of methyl ethyl ketone in a reaction vessel with stirrer,internal thermometer, reflux condenser and electrical heating. 90.7 g ofisophorone diisocyanate are then added at 45° C. After the exothermicreaction has subsided the mixture is slowly heated to 80° C. It ismaintained at this temperature until the NCO content is 1.8%. It is thencooled to 50° C., and 14.9 g of triethylamine and 535.3 g of deionizedwater are added in rapid succession. After 15 minutes a mixture of 7.6 gof aminoethylethanolamine and 19.3 g of deionized water are added to thewell-dispersed resin. The temperature is subsequently raised to 60° C.and the methyl ethyl ketone is distilled off in vacuo. The resultingdispersion has a solids content of 34.3% by weight (60 min at 130° C.)and a pH of 8.0.

514.7 g of the polyurethane resin dispersion prepared above are dilutedwith 277.7 g of deionized water. The solution is heated to 85° C. andthen a mixture of 50.1 g of styrene, 50.1 g of methyl methacrylate, 37.5g of n-butyl acrylate and 37.5 g of hydroxyethyl methacrylate is addedslowly over the course of 3.5 hours. Commencing with the addition ofthis mixture, a solution of 2.6 g of t-butyl peroxyethylhexanoate in 30g of methoxypropanol is added over the course of 4 hours. Subsequentlythe mixture is maintained at 85° C. until the monomers have reactedcompletely. Additional initiator is added if desired. Finally anycoagulated material which may have been produced is filtered off. Theweight ratio of polyurethane resin to acrylate monomers is 1:1. Theresulting dispersion displays a very good stability on storage and has asolids content of 34.8% by weight (60 min at 130° C.) and a pH of 7.2.

2. Preparation of a Basecoat According to the Invention

2.1 Preparation of a Pigment Paste

A paste is made from 40 g of a commercially available aluminum bronze(aluminum content: 65%) together with a mixture of 10 g of water, 10 gof butoxyethanol, 15 g of a commercially availablehexamethoxymethylmelamine resin and 70 g of a 31% by weight strengthpolyurethane resin dispersion prepared in accordance with the teachingof WO 92/15405.

2.2 Preparation of the Basecoat

306 g of the binder dispersion prepared according to Section 1. aremixed while stirring with 5 g of N-methylmorpholine and 79 g ofbutoxyethanol.

Then 250 g of a 3% strength commercially available thickener solutionbased on a polyacrylate dispersion are added with stirring and themixture is adjusted to a pH of between 7.3 and 7.6 withN-methylmorpholine. The pigment paste prepared according to Section 2.1is then added slowing with stirring. Thereafter 143 g of water are addedslowly with stirring (about 800 rpm). Finally the mixture is adjustedwith water to a spray viscosity of 30 s in accordance with DIN 53211.

3. Preparation of a Two-Coat Finish using a Basecoat According to theInvention

The basecoat prepared in accordance with Section 2.2 is applied, using aflow-cup spray gun, to a bodywork panel coated with a commerciallyavailable electrodeposition coating and with a commercially availablefiller, in such a way that the basecoat film, dried for 5 min at 20° C.and for 10 min at 80° C., has a dry film thickness of about 15 μm. Acommercially available 2-component clearcoat is applied over thebasecoat film dried in this way and is baked for 30 min at 130° C. Thedry film thickness of the clearcoat is about 40 μm. The two-coat finishobtained in this way has an excellent metallic effect. A number of testsin accordance with the so-called adhesive-tape tear off method show thatboth the adhesion between basecoat and clearcoat and the adhesionbetween filler coat and basecoat is excellent. To test the facility forcoating over the finish with a refinish, the basecoat prepared inaccordance with Section 2.2 is applied over the baked two-coat finishwith a flow-cup spray gun in such a way that the basecoat film, driedfor 5 min at 20° C. and for 10 min at 80° C., has a dry film thicknessof about 15 μm. A 2-component clearcoat is then coated over the basecoatfilm dried in this way and is cured for 30 min at 80° C. The refinishobtained in this way has an excellent metallic effect. After storage for240 hours at 40° C. and a relative atmospheric humidity of from 95 to100% the finish, both with and without the refinish coated over it,exhibits no blistering, swelling or matting after a regeneration phaseof one hour.

Comparative Experiment

The procedure as described under Sections 1 to 3 is repeated, with thesole exception that, instead of the binder dispersion according to theinvention prepared in accordance with Section 1, an equivalent quantityof the binder dispersion described in Example 1 of EP-A-353 797 isemployed. The stability to high atmospheric humidity of the resultingfinishes ranges from unsatisfactory to completely inadequate.

What is claimed is:
 1. A process for the preparation of a two-coatfinish on a substrate surface, comprisinga) providing an aqueousdispersion of a polyurethane resin having hydrophilic groups, the resinhaving a number-average molecular weight of from 1000 to 30,000 andcontaining, on average, from 0.05 to 1.1 polymerizable double bonds permolecule; b) polymerizing in the aqueous dispersion, and in the presenceof at least one water-insoluble initiator, at least one member selectedfrom the group consisting of ethylenically unsaturated monomers andmixtures thereof, to produce a polymer, wherein the weight ratio ofpolyurethane resin to ethylenically unsaturated monomer is from 1:10 and10:1; c) preparing a pigmented aqueous basecoat containing the polymerof step b) as binder; d) applying the pigmented aqueous basecoat to thesubstrate surface; e) forming a polymer film from the basecoat appliedin step d); f) applying a transparent topcoat to the basecoat and; g)baking and curing the basecoat and the topcoat together.
 2. A processaccording to claim 1, wherein the polyurethane resin contains groupshaving polymerizable double bonds selected from the group consisting ofacrylate, methacrylate, allyl ether groups, and mixtures thereof.
 3. Aprocess according to claim 1, wherein the polyurethane resin is anionicand has an acid number of from 20 to
 60. 4. A process according to claim1, wherein the ethylenically unsaturated monomers employed are a mixtureof(i) from 40 to 100% by weight of an aliphatic or cycloaliphatic esterof acrylic acid or methacrylic acid, which contains neither hydroxyl norcarboxyl groups, or of a mixture of such esters, (ii) from 0 to 30% byweight of an ethylenically unsaturated monomer which carries at leastone hydroxyl group in the molecule, or of a mixture of such monomers,(iii) from 0 to 10% by weight of an ethylenically unsaturated monomerwhich carries at least one carboxyl group in the molecule, or of amixture of such monomers, (iv) from 0 to 50% by weight of anethylenically unsaturated monomer which is different from (i), (ii) and(iii), or of a mixture of such monomers, and (v) from 0 to 5% by weightof an ethylenically polyunsaturated monomer or of a mixture of suchmonomers, the sum of the proportions by weight of (i), (ii), (iii), (iv)and (v) always being 100% by weight.
 5. Aqueous coating materialcomprising as binder a polymer which is obtained by subjecting a memberselected from the group consisting of ethylenically unsaturated monomersand mixtures thereof to free radical polymerization in an aqueousdispersion of a polyurethane resin in the presence of at least onewater-insoluble initiator;wherein said polyurethane resin hashydrophilic groups and a number-average molecular weight of from 1000 to30,000 and contains on average from 0.05 to 1.1 polymerizable doublebonds per molecule, and further wherein the weight ratio of thepolyurethane resin to the ethylenically unsaturated monomer or to themixture of ethylenically unsaturated monomers is between 1:10 and 10:1.6. Aqueous coating materials according to claim 5, wherein thepolyurethane resin contains groups having polymerizable double bondsselected from the group consisting of acrylate, methacrylate, allylether groups, and mixtures thereof.
 7. Aqueous coating materialsaccording to claim 5 wherein the polyurethane resin is anionic and hasan acid number of from 20 to
 60. 8. Aqueous coating materials accordingto claim 5, wherein the ethylenically unsaturated monomers employed area mixture of(i) from 40 to 100% by weight of an aliphatic orcycloaliphatic ester of acrylic acid or methacrylic acid, which containsneither hydroxyl nor carboxyl groups, or of a mixture of such esters,(ii) from 0 to 30% by weight of an ethylenically unsaturated monomerwhich carries at least one hydroxyl group in the molecule, or of amixture of such monomers, (iii) from 0 to 10% by weight of anethylenically unsaturated monomer which carries at least one carboxylgroup in the molecule, or of a mixture of such monomers, (iv) from 0 to50% by weight of an ethylenically unsaturated monomer which is differentfrom (i), (ii) and (iii), or of a mixture of such monomers, and (v) from0 to 5% by weight of an ethylenically polyunsaturated monomer or of amixture of such monomers, the sum of the proportions by weight of (i),(ii), (iii), (iv) and (v) always being 100% by weight.